1. Field of the Invention
The present invention relates generally to radioisotope labeled prostate specific membrane antigen (PSMA) binding compounds, chemical precursors of radioisotope labeled PSMA binding compounds and imaging methods using the radioisotope labeled compounds.
2. Background
Prostate cancer (PCa) is the second leading cause of cancer-related death in men (1). Only one half of tumors due to PCa are clinically localized at diagnosis and one half of those represent extracapsular spread. Localization of that spread as well as determination of the total body burden of PCa have important implications for therapy, particularly as new combination and focal therapies become available. Also critically needed are targeted agents that can provide a readout on the biology of the tumor, with the ability to predict which tumors will lie dormant and which will develop into aggressive, metastatic disease. The current clinical standard for localizing cancer—including PCa—is shifting from the anatomic techniques such as computed tomography (CT) and magnetic resonance (MR) imaging to more physiologically relevant methods that employ molecular imaging, such as MR spectroscopy, single photon emission computed tomography (SPECT) and positron emission tomography (PET) (2). Such newer methods that utilize molecular imaging may provide the biological readout necessary for understanding tumor physiology, enabling more accurate prognosis and therapeutic monitoring. Molecular imaging may provide a way to not only detect tumors in vivo, but also to provide information regarding the biology of the lesion, if a mechanism-specific agent is used. For example, [18F]FDHT can be used to study the androgen receptor status of tumors (3).
Unlike many other cancers, PCa is particularly difficult to detect using existing molecular imaging tracers. There are several reasons for this, including the relatively slow growth and metabolic rate of PCa compared to other malignancies as well as the small size of the organ and proximity to the urinary bladder, into which most radiopharmaceuticals are eventually excreted.
Because of the relatively low metabolism of PCa, PET with [18F]fluorodeoxyglucose (FDG-PET) has proved ineffectual for diagnostic imaging of this disease. Other promising, experimental radiopharmaceuticals for imaging PCa are emerging, including those of the choline series (4)(5)(6), radiolabeled acetates (7), anti-1-amino-3-[18F]fluorocyclobutyl-1-carboxylic acid (anti[18F]F-FACBC) (8)(9), 1-(2-deoxy-2-[18F]fluoro-L-arabinofuranosyl)-5-methyluracil ([18F]FMAU) (10) and [18F]fluorodihydrotestosterone ([18F]FDHT) (3). Each has its benefits and detriments, with no single agent ideal, i.e., easy to synthesize, little metabolism and demonstrating tumor-specific uptake, in all PCa phenotypes.
Overexpressed on most solid tumor neovasculature (11) as well as in prostate cancer, the prostate-specific membrane antigent (PSMA) is becoming an attractive target for cancer imaging and therapy (12)(13). PSMA-based agents can report on the presence of this marker, which is increasingly recognized as an important prognostic determinate in PCa (14). It is also the target for a variety of new PCa therapies (15). ProstaScint™ is an 111In-labeled monoclonal antibody against PSMA that is clinically available for imaging PCa. ProstaScint™ and radiolabeled variations of this antibody are fraught with long circulation times and poor target to nontarget tissue contrast, limiting the utility of these agents (16)(17)(18).